Gasoline specifications are becoming stricter and more difficult for refiners to meet. For example, it is difficult for hydrocracker based refineries to meet the aromatics specifications in the Euro-V gasoline standard while maximizing 95 RONC (research octane number clear) without having a heavy naphtha export stream. For example, certain standards may limit gasoline to concentrations of no more than 35 lv % aromatics; concentration of no more than 1.0 lv % benzene; distillation specifications and Reid vapor pressure (RVP) limit etc. The heavy naphtha stream has lower value, thus reducing the refiner's profitability.
A typical hydrocracker based refinery naphtha block includes a C5-C6 isomerization zone and a catalytic reforming zone. In order to minimize aromatics production, C7 needs to be removed from the feed to the catalytic reforming zone. This can be done with a second naphtha splitter or a side draw from a naphtha splitter, for example. Although this approach minimizes the amount of aromatics produced from Cis, it does not allow for 95 RONC gasoline production due to the low blending octanes of components in the C7 stream when blending directly to gasoline pool.
A solution is to use a single stage C7 isomerization zone with a large recycle stream to maximize the octane of the isomerate. In order to maximize the octane, a deisoheptanizer (DIHP) column is used to produce an overhead stream, a side cut stream, and a bottom stream. The overhead stream primarily comprises high octane multi-branched C7 hydrocarbons. The side draw stream is a mixture of single-branched, normal, and cycloalkane C7s. This is a lower octane stream and is recycled back to reactor to be converted to multi-branched C7s. The bottom stream comprises n-heptane, C7 cycloalkanes and heavies. In order to achieve a high proportion of 95 RONC gasoline, this C7 isomerization zone configuration results in very high operating costs due to large recycle stream and a lack of on-stream flexibility due to the single isomerization stage.
Therefore, there is a need for a more flexible process of making gasoline with an increased amount of 95 RONC.